An optical transmission system transmits information from one place to another by way of a carrier whose frequency is in the visible or near-infrared region of the electromagnetic spectrum. A carrier with such a high frequency is sometimes referred to as an optical signal, an optical carrier, or a lightwave signal. Such optical signals are commonly propagated in optical fibers.
In some systems, photonic devices are built into or include a segment of optical fiber. For example, a Bragg grating can be implemented in a segment of optical fiber by forming regions of periodically alternating refractive indices in the fiber segment through which an optical signal is propagated. This type of Bragg grating is commonly referred to as a fiber Bragg grating (FBG) and is typically used as a wavelength selective filter in fiber optic communication systems. For example, the FBG can be used to filter out a particular wavelength (known as the Bragg wavelength). The Bragg wavelength depends on the average or effective refractive index of the optical fiber segment and on distance between gratings of the alternating regions (i.e., the period). As is well known, the Bragg wavelength of a FBG is dependent on the temperature and the strain on the fiber segment containing the FBG.
Typically, the optical fiber segment containing the FBG is attached under strain to a package that can be mounted on a board or otherwise incorporated into a unit or assembly. As schematically illustrated in FIG. 1 (PRIOR ART), the optical fiber segment is attached to a conventional package at two attachment points, with the FBG section being between the attachment points.
However, the size of the package can be sensitive to temperature. For example, the package can expand when the temperature changes, which in turn can change the strain applied to the optical fiber segment containing the FBG. This temperature dependent strain induced by the package can be used to augment or compensate the intrinsic temperature sensitivity of the FBG. Packages wherein an increase in temperature leads to a decrease in strain can have a reduced temperature sensitivity of the Bragg wavelength. Such packages are commonly referred to as athermal packages.
One conventional athermal package is schematically illustrated in FIG. 1 (Prior Art). As shown, an optical fiber 100 containing a FBG region 101 is attached to a package. The package has portions 102 and 103 that are made of one material and a portion 106 made of another material. Optical fiber 100 is attached under strain to portions 102 and 103 using bonds 104 and 105, with FBG region 101 being between the bonds. Bonds 104 and 105 are commonly solder, epoxy or other adhesive. Generally, such conventional solutions select the materials of portions 102, 103 and 106 so that when a temperature change occurs that causes portion 106 to expand (i.e., increasing the strain on the segment of optical fiber 100 between bonds 104 and 105), portions 102 and 103 expand so as to reduce the strain on the segment of optical fiber 100 between bonds 104 and 105. Ideally, the expansion of portions 102 and 103 exactly counteract the increase in strain caused by expansion of the portion 106 and the inherent temperature dependence of the FBG. Precise control of the dimensions of the package and the length of the optical fiber between bonds 104 and 105 are needed to achieve the desired temperature compensation of the package. However, over large temperature ranges, non-linear thermal dependencies can detrimentally affect the performance of such conventional packages because the designs are based on assuming of a linear temperature dependence of the package and fiber containing the FBG.